“外键"与“外键"之间的区别在于:和“约束外键"；

问题描述：

我的意思是，例如，我可以创建表

I mean for example I can create table like

create table XTable
( 
  idt int not null primary key,
  value nvarchar(50),
  idq int,
  constraint fk_idq foreign key(idq) references YTable(idq)
)

我可以这样创建它

create table XTable
(
  idt int not null primary key,
  value nvarchar(50),
  idq int,
  foreign key(idq) references YTable(idq)
)

我通常像第二个示例中那样创建表，但是现在我对第一个示例感到好奇.有什么区别?

I usually create table like in the second example but now I'm curious about the first example. What is the difference?

答

第一个选择纯粹是为了命名约束.

The first option is purely for naming the constraint.

来自 SQL外键约束

要允许命名FOREIGN KEY约束并在多个列上定义FOREIGN KEY约束，请使用以下SQL语法

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax

CREATE TABLE Orders
(
  O_Id int NOT NULL,
  OrderNo int NOT NULL,
  P_Id int,
  PRIMARY KEY (O_Id),
  CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
  REFERENCES Persons(P_Id)
)

此外，来自创建表(交易记录-SQL)可以看到[ CONSTRAINT constraint_name ]是可选的.

Also, from CREATE TABLE (Transact-SQL) one can see that [ CONSTRAINT constraint_name ] is optional.

“外键"与“外键"之间的区别在于:和“约束外键"；

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